In the printing industry, different types of sheet inverters are known, which are typically employed in a duplex path of a printing machine. One such type of sheet inverter, which is for example described in U.S. Pat. No. 6,626,103 B1, allows a leading edge of a sheet to remain the same before and after inversion. This may be beneficial to registered printing on the front and back side of the sheet.
This known sheet inverter has a first pair of spaced apart rollers having a first endless belt entrained thereabout in a figure eight configuration such that the first belt has a 180° twist in each section extending between the first pair of rollers and a second pair of spaced apart rollers having a second endless belt entrained thereabout in a figure eight configuration such that the second belt has a 180° twist in each section extending between the second pair of rollers. The first and second pairs of rollers are arranged adjacent to each other such that one section of the first belt and one section of the second belt, each belt having the 180° twist, contact each other to form a transport path therebetween. This is done by arranging one roller of the first pair of rollers and one roller of the second pair of rollers adjacent to each other to form an entrance group of rollers at an entrance of the transport part, and arranging one roller of the first pair of rollers and one roller of the second pair of rollers adjacent to each other to form an exit group of rollers at an exit of the transport path. The transport path thereby also has a 180° twist. The twist forces a sheet sandwiched and transported between the belts to be twisted along the line of contact and to turn by 180° while it is transported along the transport path through the inverter. The inverter also has a guide wire to guide an edge portion of the sheet over and across the transport path during turning of the sheet. The known guide wire is arranged to support the edge portion of the sheet in substance in accordance with the turning force impaired by the twist in the transport path, i.e. it is arranged to provide substantially the same turning rate as the one provided by the twist in the transport path.
This arrangement is suitable for a wide range of sheets, which may differ with respect to stiffness and dimensions. For long sheets, however, in particular sheets having a length (in the direction of transportation) longer than half the length of the transport path through the inverter, this arrangement may cause problems as described below. In this case, when the leading edge reaches the midpoint of the transport path, both the twist in the transport path and the guide wire will urge the sheet into an upright orientation. While the transport path urges the sheet in an upright orientation only in a middle section thereof, the guide wire urges the edge portions thereof in the upright orientation. When the sheet is longer than half the length of the transport path, the trailing edge will still be held in a horizontal position between the entrance group of rollers at the entrance end of the transport path. This may cause jams, undue stress in the sheet and may especially lead to artifacts in a printed image on a surface of the sheet due to excessive bending thereof. This problem is obviously more pronounced the longer and the stiffer the sheet.
It is therefore an object of the invention to provide a sheet inverter and a method for inverting a sheet in a printing machine, which may overcome or at least lessen one of the above problems.